Venmo doesn’t use ACH when moving from Venmo to Venmo. And is the same with FedNow. If you send money from BankA account to bankB account, that’s an ACH transaction and it’s recorded as such. Same with Fed now.
Right, these concerns are predicated on apps built natively on FedNow gaining market share to rival Venmo enough that these transactions are no longer opaque. If everybody uses it the same way they use ACH, there's no difference to the state of the world today.